What speakers mean now
In ordinary English, an idea is a thought with some outline: a plan, an opinion, a notion of how a thing works, or only a faint sense that something is the case. You can float an idea, steal one, or have no idea. The noun also covers the gist of a situation and, in music, a theme. Alongside that everyday range sits a narrower philosophical job. There the word names an abstract archetype, a pattern that particular things are said to copy only imperfectly.
Those two jobs grew from a single learned borrowing. English did not start with the casual mental sense and later dress it up for the schools. It started in the schools and spent centuries letting the noun walk out into common speech.
A Latin word for a Greek Form
The English noun is a borrowing from Latin idea. Latin had taken it from Ancient Greek ἰδέα, where it meant form, appearance, kind, nature, or manner: the look of a thing, and then a class or species in logical talk. Plato made the same noun a name for an immaterial pattern. Objects in the visible world, on that account, have their being by participating in the pattern; they are copies, not the original.
Latin writers of philosophy adopted the Greek term rather than inventing a native equivalent. Cicero often left the word in Greek letters instead of treating it as fully Latin. Later Latin idea became the usual label for that Platonic archetype. English took the Latin form in the late fourteenth century as a technical word of theology and philosophy, not as a household noun.
The earliest securely cited English use falls before 1398, in a translation by John Trevisa. Late Middle English idea meant something close to an archetype, including the concept of a thing as it exists in the mind of God. That is the door through which the word entered. The mental picture you carry of a chair, or the scheme for rearranging a kitchen, is a later widening of the same form.
The path is borrowing, then, and a learned one: Greek into Latin into English, carried by school Latin rather than by Old English inheritance. French idée is a doublet, the same Latin source reshaped in French. Through Latin the word also reached Dutch, German, and Danish as idee, a shorter shape that turns up in some English dialects.
Built on a verb of seeing
Greek ἰδέα belongs with the infinitive ἰδεῖν, to see, and with the verb family of εἴδω. Behind them sits a Proto-Indo-European root reconstructed as *weid- or *weyd-: to see, and by a very old extension, to know. Some accounts posit a suffixed form *wid-es-ya- for the Greek noun. That string is a reconstruction, not a word anyone wrote down, and the exact PIE shape can be stated in more than one way. The broader family is not in serious dispute.
Greek eidenai, to know, sits on the same root. So does eidos, form or kind, which later English used in idol, eidolon, and the suffix -oid. Latin videre, to see, supplies vision, view, video, and a long list of verbs of looking, reviewing, and providing. Native Germanic descendants include Old English wit and wise, and German wissen, to know. Sanskrit veda, I know, is a further cousin. Lithuanian and Slavic languages keep verbs of seeing from the same stock.
The morphology is therefore real, not a folk tale: a noun of appearance formed from a verb of seeing. The old equation is that what has been seen is, in some sense, known. Modern idea no longer sounds like a word of the eyes, but its relatives still do. There is no need to invent a tidy story beyond that. Where the PIE reconstruction is uncertain, it is better left as a sketch than forced into a single ancestral word.
From archetype to plan
For a long stretch the English noun stayed in the school. It named the Platonic Form or a divine pattern. From the sixteenth to the eighteenth century it could also mean the form or shape of something, a quintessential aspect; that sense is now obsolete. Another obsolete use, listed in later dictionaries, is a mental image of something remembered.
The senses that dominate today are Early Modern. By the 1610s idea could mean a picture in the mind. The same decade yields a plan, a concept of something to be done, and a standard of what ought to be, set against what is observed. By the 1640s the noun could mean simply the product of thinking. A vaguer use, a hunch or impression, is also seventeenth-century.
Greek had other words for the act of thought, among them ennoia. The English widening is a later development of the borrowed noun, not a straight transfer of those Greek terms. From the seventeenth century onward, Descartes, Locke, Berkeley, Hume, and Kant each assigned idea a slightly different technical role in accounts of mind and reason: representation, impression, concept of reason, and more besides. Ordinary usage kept the looser range: thought, opinion, plan, inkling.
Later English also took in French idée fixe, a fixed idea, from 1836, and the colloquial big idea from 1908, the challenge that asks what someone is really driving at. Learned offshoots such as ideal, ideology, and the combining form ideo- keep the Greek look of the base even when idea itself has gone fully common.
The hinge that still shows
If one fact is worth keeping, it is the verb under the noun. Greek speakers named a form by how it looked. English speakers now name a thought. The family still joins those acts: vision for what is seen, wise and wit for what is known. The origin of the word idea is that older hinge, where seeing was already a way of knowing.